Examining the relationship between rainfall and water table position in grassland peat soils

The artificial drainage of carbon-rich peat soils is a common practice to increase agronomic production on waterlogged lands but may lead to the release of carbon dioxide to the atmosphere. In Ireland, there are an estimated 300-350,000 ha of permanent grassland on peat soils, with varying degrees of drainage. 80,000 ha of these grassland peat soils are targeted in the Irish National Climate Action Plan for reduced management intensity which involves manipulating the water table by removing and blocking existing artificial drainage features. This process is often referred to as ‘active water table management’ or ‘rewetting’.

Actively managing the water table in grassland peat soils is an important tool to reach EU climate neutrality goals by 2050 because the water table position dictates the carbon storage dynamics of the soils. Research shows that raising the water table in these grassland peat soils by 10 cm can reduce overall greenhouse gas emissions from them. However, to achieve this, the impact that peat soil formation and subsequent anthropogenic activities (e.g., drainage and peat extraction) has had on the hydrology of these lands must be better understood.

The Irish Department of Agriculture, Food and the Marine-funded project, ReWET, aims to provide a deeper understanding of the hydrologic impacts of active water table management on grassland peat soils. An objective of this project is to investigate rainfall and water table relationships at agricultural grassland sites on peat soils to: (1) compare these relationships within and across peat classification types, and (2) determine field scale hydrological patterns that can be used to aid in the classification of these and other sites into fen or raised bog peat types to establish future restoration potential. For this study, six field sites on grassland farms were selected and classified into peatland type based on their soil characteristics. The sites were instrumented with rainfall gauges and dipwells with pressure sensors to record the water table position every 15 minutes and were monitored from September 2023 through August 2024.

Results from this study show that hydrologic differences between and within peat classification types exist. For each site the annual average water table depth demonstrated that peat soil type has an impact on the drainage depth and that fen peat sites were more deeply drained than raised bog sites despite similar surface drain design. Rainfall event-based analysis allowed the sites to be compared based on total rainfall depth, water table rise, lag time from the start of an event to the highest water table position and calculated specific yield. The event-based analysis was also used to correlate water table rise with rainfall at each site and for each peat classification type. It was found that, overall, the fen sites exhibited a stronger correlation between water table rise and rainfall than the raised bog sites. The fen sites also had larger average water table fluctuations, longer average lag times and smaller average calculated specific yields during events than the raised bog sites.